A bathroom fan has one job: pull steam and odors out of the room before they settle into drywall, paint and grout. Picking the right one comes down to four things: how much air it moves, how loud it is, how it connects to your ductwork, and whether it fits the opening you already have.

1. Size the fan to the room

Fan capacity is rated in CFM (cubic feet per minute). The Home Ventilating Institute recommends:

• Bathrooms up to 100 sq ft: about 1 CFM per square foot of floor. A 5 x 8 ft bathroom (40 sq ft) needs roughly 40 to 50 CFM.
• Bathrooms over 100 sq ft: add up the fixtures instead. Allow 50 CFM each for a toilet, shower and bathtub, and 100 CFM for a jetted tub.

Long duct runs, several elbows or a high ceiling all cut into real-world airflow, so round up rather than down when you are between sizes.

2. Check the noise rating

Fan noise is measured in sones. Lower is quieter. A fan around 1 sone is easy to live with; one at 3 or 4 sones is the kind people switch off early, which defeats the purpose. If the fan will run while someone is getting ready in the morning, noise matters as much as airflow.

3. Match the duct

Look at the duct connector on your current fan. Choose a fan with the same connector size, or plan for the adapter you need. Avoid stepping down to a smaller duct, since that chokes the airflow you paid for. Keep the run as short and straight as you can, and make sure it ends outside the house. Venting into an attic just moves the moisture problem somewhere you cannot see it.

4. Replacing an old fan? Measure the housing

Most replacements are easiest when the new housing fits the existing ceiling opening. Before ordering:

• Take off the grille and measure the housing opening (length and width) and its depth.
• Note which way the duct connector points and its diameter.
• Check the wiring. A fan with a built-in light usually needs a separate switch for the light if you want to run them independently.

Send us those measurements and a photo of the old fan's label, and we can help you pick a KSTAR model that fits.

5. Use it the right way

Turn the fan on when you start the shower and leave it running for about 20 minutes afterward. That is HVI's guidance, and it is what keeps mirrors clear and mildew out of the corners.

Buying for more than one bathroom?

Contractors, landlords and anyone doing several bathrooms at once can save with our 6-packs.

Browse KSTAR ventilation fans, or call (800) 893-2168 or email info@kstarus.com with your room size and existing fan details.